As different from single chip packages, multichip packages interconnect several semiconductor die. In the case of 2D (two-dimensional) based multichip modules (MCM), chips or die are interconnected on a substrate using flip chip or wire bond interconnects. Some 3D interconnects use through silicon vias (TSVs) on either the active silicon circuit die or passive silicon substrates. As an intermediate level, 2D interconnect structures involve a silicon substrate as an interconnect substrate (known as interposer) to provide high density interconnects, using wire bond or flip chip interconnects, sometimes known as 2.5D. Flip chip interconnects may be used to provide higher interconnect density due to the area nature of the interconnect, and provide higher frequency capability due to short electrical distances. The silicon interposer entails additional costs, and there may also be longer electrical distances between the interconnected semiconductor die.
As an alternative to 2.5D and the interposer, a different architecture, namely connecting the die face to face may be used. While the interconnecting the two die can be accomplished in a shorter electrical distance and with the elimination of the interposer substrate, the technique still interconnects the combination of the interconnected die to the outside world. While the face to face interconnecting of the two die can be done using either flip chip solder or copper microbumps, connection to the outside of the 2-die stack is accomplished using wire bond. With this technique, limitations may be encountered in the number of input/outputs (I/Os) and frequency limitations experienced by wire bonds. The flip chip solder interconnect for interconnecting to the outside world may be used when one interconnects the two face to face dies using microbumps.
The spherical nature of the flip chip solder dictates both the height and the I/O pitch, either limiting the height for higher I/O or limiting the I/O density to provide taller interconnects—height of the solder is critical so as to avoid the daughter die interfering with the bottom substrate. The bottom die also usually has to be thin enough to fit in the space between the top die and the substrate. For a typical flip chip external interconnect, the bottom die may be as thin as 50 microns (micrometers), which may entail more complex handling and higher cost. Flip chip external interconnect entails I/O density and die-thickness considerations. This technique also entails processing of copper microbumps and solder (lead-tin or lead free), a different material, by wafer manufacturers. Sometimes, this technique may encounter potential incompatibilities.